As shown in FIG. 36, a crawler belt 51 for construction vehicles etc. is composed of various parts. A crawler belt bushing 52 meshes with sprocket teeth for transmitting rotating movement from final reduction gears and functions to rotate the crawler belt 51. As construction vehicles are operated in soil and rock, crawler belt bushings need wear resistance at the inner and outer circumferential surfaces. Vehicles also travel, running over and colliding against soil and rock, crawler belt bushings need tremendously high strength and toughness. To meet these requirements, there have been proposed the following producing methods for crawler belt bushings.
(1) A case hardening steel is carburized to form very hard martensite on its inner and outer circumferential surface layers, thereby ensuring wear resistance and strength (e.g. Japanese Patent Publication (KOKOKU) No. 52-34806 (1977)).
(2) A medium carbon steel is used as a bushing material. The bushing material is thermally refined and its inner and outer circumferential surfaces are respectively induction hardened to form very hard martensite thereon. After hardened by induction hardening from the outer circumferential surface, the bushing material is induction hardened from the inner circumferential surface, so that a V-shaped hardened layer comprising tempered martensite is formed between the inner and outer hardened layers, thereby ensuring wear resistance and strength (Japanese Patent Publication (KOKOKU) No. 63-16314).
(3) A medium carbon steel, whose hardenability is carefully controlled by precise adjustment on its chemical composition, is used for the bushing. Such a steel is heated in a furnace at a temperature of 800.degree. C. or more and then rapidly cooled thereby controlling the hardened depths of the inner and outer circumferential surfaces to ensure wear resistance and strength.
FIGS. 37(a), (b) and (c) schematically show typical hardening patterns for the bushings manufactured by the above conventional methods, and FIG. 37(d) shows the distributions of hardness in the cross sections of these bushings. All of the distributions indicate that there is formed a soft layer at the wall core of the bushing.
These methods, however, reveal their own drawbacks. The carburization method (1) consumes considerable carburization time and has the economical problem of using large amounts of carburizing gas. When producing large-sized crawler belt bushings having great thickness for example, a great hardened depth is required in order to ensure strength and wear resistance, which gives rise to a decrease in productivity and to increased cost. Further, since it takes long time to carburize and heat the inner and outer circumferential surfaces, there will be formed a grain boundary oxidized layer and imperfectly hardened layer having a thickness of several tens of .mu.m, which causes a decrease in fatigue strength and impairs impact resistance properties.
The induction hardening method (2) is improved over the carburization method (1) in terms of cost, but involves thermal refining treatment in order to assure hardness prior to the induction hardening and double quenching of the inner and outer circumferential surfaces, so that this method has proved to remain a costly heating treatment. Further, when induction hardening the inner circumferential surface of a small-diameter tubular part, an inner circumference heating coil is needed and such a heating coil is difficult to manufacture. Therefore, in many cases, the inner circumferential surface of a tubular part is hardened by the above carburization treatment, resulting in high cost.
The outer circumferential surface of a bushing in use is subjected to severe wearing conditions due to soil and rock. To increase the wear life of the bushing, the quench hardened layer at the outer circumferential surface is preferably more deepened. To this end, there has been made an attempt in Japanese Patent Publication (KOKOKU) No. 63-16314 (1988). According to this publication, after the outer circumferential surface of a steel is once hardened deeply by induction hardening from the outer circumferential surface, the inner circumferential surface is shallowly hardened by induction hardening from the inner circumferential surface, and a soft layer is formed between these hardened layers by high temperature tempering. In any case, double induction hardening is involved, which is disadvantageous in terms of productivity and economy. In the method of this publication, it is necessary to control the induction hardening from the outer circumferential so as to restrict the hardness of the inner circumferential surface to H.sub.RC 40, thereby preventing quenching crack during the later induction hardening of the inner circumferential surface. For applying this method to a comparatively thin tubular part (e.g., crawler belt bushing), it is necessary to control the temperature of the inner circumferential surface with high precision, during the induction heating from the outer circumferential surface and/or to control the hardenability (DI value) of the steel material to be used. As a result, the technical difficulty in deepening the outer circumferential surface hardened layer and increased material cost are inevitable.
Japanese Patent Publication (KOKOKU) No. 1-37453 discloses a quite economical heating process for producing through hardened crawler bushings. This process uses a medium carbon steel for the bushing. While scan induction heating is carried out from the outer circumferential surface of the bushing, the bushing is cooled from the outer circumferential surface, so that the bushing is hardened across its entire thickness. For through hardening the bushing across its entire thickness by cooling from the outer circumferential surface only, high hardenability is required when this process is applied to the production of thick, large-sized crawler belt bushings, which inevitably entails an increase in cost. In addition, in view of susceptibility to quenching crack during cooling, the steel material that can be used in this process is limited to medium carbon low alloy steels having carbon contents of 0.5 wt % or less. As a result, it becomes difficult to improve the wear resistance of the outer circumferential surface of a crawler belt bushing.
The hardening process (3) has overcome the cost problems presented by the processes (1) and (2), but presents other problems. Specifically, it is necessary for this process to accurately and narrowly control the hardenability (DI value) of the steel material used, by correctly grasping the relationship between the thickness of the steel and the cooling rate. This process has a problem in the availability of materials to be used. In addition, as the thickness of the busing becomes smaller, the bushing is more through hardened across its entire thickness, causing higher tensile residual stress at the inner and outer circumferential surfaces. This could be a cause for quenching crack and a considerable decrease in fatigue strength. To avoid this problem, the DI value should be decreased and as a result, the steel materials which meet this requirement will not be commercially available. Further, if steels having small DI values are used, the resultant bushings will have variations in hardness.
The present invention has been directed to overcoming the above-described problems and therefore the prime object of the invention is to provide a crawler belt bushing and its producing method, improved over the products and processes of the above conventional carburization and induction hardening in terms of productivity and cost. This crawler belt bushing is produced by heating a tubular bushing workpiece made of steel to a quenching temperature and then quenching the workpiece in a series of quenching operation, without causing quenching crack, so that the workpiece is quench hardened across its entire thickness.
The invention also aims to provide a long wear life, tough crawler belt bushing and its producing method, improved over the products and processes of the above conventional carburization and induction hardening in terms of productivity and cost. This bushing is produced by heating a tubular bushing workpiece made of steel to a quenching temperature and then applying a series of quenching operation to this workpiece, the operation comprising: advance cooling of the workpiece from the inner circumferential surface and cooling of the workpiece from the outer circumferential surface after waiting a certain time. With this process, quench hardened layers are formed at the inner and outer circumferential surfaces respectively, such that at least the depth of the outer circumferential surface hardened layer is greater than that of the inner circumferential surface hardened layer.
The invention provides a crawler belt bushing producing method applicable to inexpensive steel materials having higher commercial availability than the above-described through-hardening process from the outer circumferential surface only, by achieving through hardening by cooling from both inner and outer circumferential surfaces although there is a difference between the starting time for the inner circumferential surface cooling and the starting time for the outer circumferential surface cooling.
The invention also aims to provide a crawler belt bushing and its producing method, improved over the products and processes of the above conventional carburization and induction hardening in terms of productivity and cost, this bushing being produced by the following process. A tubular bushing workpiece made of steel is heated by scan induction heating from its outer circumferential surface such that at least the temperature of the inner circumferential surface of the workpiece is raised to a quenching temperature. Then, a series of quenching operation is carried out as follows. While firstly starting cooling from the inner circumferential surface, the outer circumferential surface is heated by induction heating to restrict the cooling of the outer circumferential surface occurring from the inner circumferential surface. After waiting a certain time in order to prevent the core of the workpiece from being fully hardened, cooling from the outer circumferential surface is started. Through these steps, quench hardened layers are formed at the inner and outer circumferential surfaces respectively.
For ensuring formation of a quench hardened layer at the inner circumferential surface, the advance cooling from the inner circumferential surface, the induction heating from the outer circumferential surface and the later cooling from the outer circumferential surface are carried out as described earlier, thereby forming a soft layer within the core of the bushing at a cross-sectional position closer to the inner circumferential surface. With this arrangement, a soft layer can be formed in the core, even when using, as a bushing material, a steel having such a great DI value that the steel is through hardened by cooling from the inner circumferential surface only. In consequence, quenching crack can be prevented and the hardened depth of the outer circumferential surface can be greater than the hardened dept of the inner circumferential surface, so that a crawler belt bushing improved in wear resistance and fatigue strength and its producing method can be provided.
The above induction heating/hardening method of the invention is applicable to tubular parts similar to crawler belt bushings. Since this method does not need to carry out heat hardening from the inner circumferential surface and therefore does not use an induction heating coil for an inner circumferential surface, small-diameter tubular parts (e.g., small-sized crawler belt bushings) and thin tubular parts (e.g., crawler belt bushings) can be produced at low cost.
Further, since a soft layer can be formed within the wall of the crawler belt bushing and quenching crack can be prevented even when using a steel having high hardenability as has been noted above, the invention can provide a crawler belt bushing having high wear resistance at its outer circumferential surface and its producing method, by hardening a steel material which has comparatively high hardenability and a carbon content of about 2.0 wt % and is composed of austenite containing cementite grains dispersed therein. It is preferable to substantially uniformly disperse cementite in the workpiece by thermal refining or the like, prior to starting of the hardening operation.